A peculiarity of cellular mobile communications networks is that they feature a plurality of so-called “network cells”, where by the term cell there is intended the set of geographical points (“pixels”, in jargon) covered by the radio electromagnetic signal irradiated by a common antenna. Cellular mobile communications networks thus provide coverage of a determined geographic region by means of the plurality of network cells.
Among the different type of cellular mobile communications networks, some networks have a radio access front end exploiting the CDMA access scheme to the shared (radio) communication medium. This is for example the case of third-generation cellular mobile communications networks, currently starting to be deployed. One of the third-generation mobile communications standards is the so-called Universal Mobile Telecommunications System (UMTS), which is in particular the standard that has been adopted by operators in Europe.
CDMA is a technique of accessing a shared communications medium according to which a same frequency bandwidth (a “channel”) is assigned simultaneously to all the requesting users; the discrimination among different signals intended for different users is accomplished by exploiting a coding scheme, according to which different codes are assigned to different users, and the signals directed thereto are coded using the respective codes. The codes assigned to different users and exploited for coding the signals directed thereto need to be “orthogonal”. The coding process comprises a so-called “spreading” operation, according to which the bandwidth of the original (uncoded) signal is widened, in particular spread over a larger bandwidth, at the same time reducing the average signal power. The spreading is achieved by coding the signal using a code that contains a higher number of symbols than the number of bits to be transmitted; the coded signal thus has a symbol rate (the so-called “chip rate”) higher than the bit rate of the original signal.
A “scrambling” process is further implemented, by applying a “scrambling code” to the signal after the spreading operation, with the purpose of scrambling the different symbols. The scrambling operation does not increase the signal bandwidth (the symbol rate is not changed compared to the chip rate of the spread signal), and can be viewed as the addition of a “color” to the signal, that allows identifying the transmission source. Particularly, in downlink (i.e., from the radio base station to the mobile terminal), the scrambling process allows distinguishing the signals within a given network cell from the signal within a different cell: to this purpose, different scrambling codes are used in different cells, in particular if such cells are neighboring.
The adoption of the CDMA access scheme has an impact on the “handover” procedures, by which, generally speaking, there is intended the set of procedures that makes it possible to keep active a service provided to a generic mobile user even when the user moves. In particular, in the CDMA access scheme a mobile user may exploit a same radio channel in different cells; thus, the passage of responsibility (handover) of a given mobile user from one network cell to another adjacent thereto (typically, in consequence of the movement of the mobile user through the geographic area) can be handled by keeping the communication with the user active on the same channel. In particular, thanks to the fact that the signals issued from different sources (different antennas corresponding to different cells) are distinguishable because of the use of different scrambling codes, a mechanism referred to as “soft-handover” (relying on a particular type of receiver in the User Equipment—UE—, called “Rake”) allows the mobile user's terminal to decode signals coming from, and thus to exchange information with, two or more different antennas or, more generally, with different radio base stations. In particular, thanks to this “soft-handover” mechanism, the UE can distinguish between signals issued by different cell radio base stations by looking at the different signal color. This takes place in particular areas, referred to as “soft-handover areas” or “macrodiversity areas”. The different network cells to which the UE is simultaneously connected form the so-called “active set”.
As known, in the UMTS the set of scrambling codes used in downlink is represented by the Gold codes featuring low self-correlation and cross-correlation. The length of the Gold code for the UMTS system is in principle equal to eighteen bits, for a total of 218−1=262,143 different codes. However, in order to keep the receiver not too complex, only a fraction of such a vast set of codes is effectively exploited in practical implementations. Specifically, the standard prescribes that the number of usable codes in UMTS networks is limited to a pool of 8,192 different codes. The pool of 8,192 usable scrambling codes is subdivided into 512 groups, each group including 16 codes, where one of the sixteen codes takes the role of a so-called “primary scrambling code”, and the remaining 15 codes of the group are “secondary scrambling codes”.
When planning an UMTS network, or a particular regional area thereof, a unique primary scrambling code (and, consequently, the associated 15 secondary scrambling codes associated to that primary scrambling code), chosen among the available 512 primary scrambling codes, has to be assigned to each cell of the area under planning.
The pool of 8,192 scrambling codes available for use is further considered as subdivided into 64 code groups of 128 codes each, where, within the generic code group, eight codes among the 128 codes are primary scrambling codes; thus, the pool of 8,192 available scrambling codes includes 64 code groups, each one including eight primary scrambling codes (and associated secondary scrambling codes).
In downlink the primary scrambling code plays a role in the procedure called “cell search”, which includes the set of operations that allow the UE synchronize to the network and decode the control channels of the network cell wherein it is located. Specifically, the UE invokes the cell search procedure in either one of two cases:                whenever the UE is turned on and has to register to the network for the first time (after a previous de-registration in consequence to a turn off); or        for purposes of measuring the common channels of the adjacent cells, with the aim of updating the so-called “active set” of different network cells to which the mobile terminal is connected (a procedure called “cell reselection”).        
The cell search procedure impacts the UE performance: depending on the complexity of the operations to be performed, the UE battery charge consumption, as well as the time required by the UE for synchronizing and decoding the control channel (the so-called “Broadcast Control Channel”—BCH) over which the network information travels vary. In particular, the cell search procedure impact on the battery charge consumption is higher when the procedure is performed in support of the cell reselection procedure, because such operation is carried out more frequently compared to the initial synchronization of the UE upon turning it on.
The assignment of scrambling codes in downlink can be effected by means of planning algorithms, such as the one described in Y. H. Jung and Y. H. Lee, “Scrambling code planning for 3GPP W-CDMA systems”, IEEE VTC2001 Spring, Rhodes, Greece, May 2001.
In particular, the scrambling code assignment has to satisfy a primary scrambling code re-use requirement, according to which unique primary scrambling codes, within the set of 512 available primary scrambling codes, have to be univocally assigned to neighboring cells belonging to the geographic area being planned. In particular, two generic cells are considered neighboring cells if at least one pixel of a cell “touches” a pixel of the other cell (geometrical neighborhood), or, according to a different criterion, if there exist portions of a cell wherein the power level of a predetermined channel, particularly the CPICH (RSCP) pilot channel of the other cell exceeds a predetermined threshold (electromagnetic neighborhood).
As it can be derived from the discussion made in S. Kourtis, “Code Planning Strategy for UMTS-FDD networks”, in Proceedings of VTC 2000, Tokio, Spring 2000, the distance between primary scrambling codes assigned to neighboring network cells impacts the performance of the UE, directly affecting the computational cost of the cell search procedure, and the synchronization time of the UE, at the frame level. In particular, with the increase of the number of scrambling codes groups assigned to adjacent cells, within the pool of 512 scrambling codes, the time needed for achieving synchronization rises, whereas the computational complexity is reduced. On the contrary, with the decrease of the number of scrambling codes groups assigned to adjacent cells, within the pool of 512 primary scrambling codes, the time needed for achieving synchronization decreases, but the computational complexity increases.
The choice of which strategy to adopt is not univocal, and depends on the considered scenario. According to Kourtis, in a case of a micro-cell network (i.e., a network having cells of limited area coverage), typical of a urban environment, it is important to minimize the time required by the cell search procedure, particularly if the UEs exhibit a medium/high mobility: a poor synchronization performance means that the number of measurements taken by the UE per measuring period is small, thus the UE may not have the necessary information required to perform the soft-handover efficiently.
In the case of a macro-cell network (with network cells of relatively wide area coverage), more typical of sub-urban and rural environments, the greater cells' dimension allows tolerating higher delays in the cell reselection procedure. Then, it is preferable to privilege strategies that minimize the computational cost of the mobile terminal, thus increasing the battery charge life, also in view of the fact that, on average, the transmitted power of the terminals is higher in macro-cell environments than in micro-cell ones.
Kourtis provides guidelines for planning the primary scrambling codes in downlink; in particular, the guidelines are given providing some primary scrambling codes planning configurations which are to be preferred, and which are expressed in terms of the number (M) of different code groups among which the primary scrambling codes to be assigned to neighboring network cells are chosen, and the number (L) of primary scrambling codes within each code group.